Variable slit systems for mass spectrometer ion sources



'June 1, 1965 R. D. CRAIG ETAL, 3,187,179

VARIABLE SLIT SYSTEMS FOR Mass SPEGTROMETER ION SOURCES Filed Aug. 30. 1962 2 Sheets-Sheet 1 Edward WIY/d/y Frank Edmund 5 ancraff wwaww ATTORNEY June 1, 1965 R. D. CRAIG ETAL 3,187,179

VARIABLE SLIT SYSTEMS FORMASS SPEQTROMETERION SOURCES Filed Aug. 50, 1962 2 Sheets-Sheet 2 55 o 64 m 2 53 L 5' T 52 f :I 59 7/ film IIIII-Il ai x.

. INVENTORS: 5/ a2 Robert Derek Crai Edward Wl'l/d/Q Frank Edmund Bancroft ATTORNEY United States Patent 3,187,179 VARIABLE SLIT SYSTEMS FOR MASS SPECTROMETER ION SOURCES Robert Derek Craig, Bowdon, Edward Willdig, Stretford, and Frank Edmund Bancroft, Sale, England, assignors to Associated Electrical Industries Limited, London, England, a British company Filed Aug. 30, 1362, Ser. No. 220,411 Claims priority, application Great Britain, Sept. 4, 1961,

Claims. (Cl. 250-419) The present invention relates to mass spectrometer ion sources in which a sample to be analysed is first ionised.

The sample may be a gas, a liquid or a solid. The ions produced in the ion source are subsequently accelerated from the ion source into the main body of the mass spectrometer for analysis in any well known manner.

The ions are conveniently accelerated through a slit in the end wall of the ion source into a channel which connects with the analyzer section of the mass spectrometer.

The slit is used to determine the resolution and sensitivity obtainable in the spectrometer and its width is varied to obtain a compromise between resolving power and sensitivity.

The object of the present invention is to provide an ion source for a mass spectrometer with an improved form of variable slit.

According to the present invention a mass spectrometer ion source comprises a chamber adapted to be evacuated, means for producing a beam of ions from a sample located within said chamber, a slit in the wall of said chamber connecting with a channel which leads to the analyzer section of the mass spectrometer, means for causing said beam of ions to pass through aid slit and along said channel, and means located outside said chamber for mechanically varying the width of said slit.

Preferably said slit is defined by the adjacent edges of two plates adapted to move towards one another across the wall of the chamber so as to vary the width of said slit, and means are provided outside said chamber for controlling the relative positions of said plates.

The means for varying the relative positions of said plates may comprise a plunger adapted to move between said plates so as to. force said plates apart .and controlled from outside said chamber.

In order that the invention may be more readily understood reference will now be made to the accompanying drawings, in which:

FIG. 1 is a side view partly sectioned along an axial plane of an ion source embodying the invention;

FIG. 2 is a view partially cut away, on a larger scale,

2 sectioned on the plane 111-411;

FIG. 4 is a plan view of another variable slit device;

3,187,179 Patented June 1, 1965 ice ment, an electron accelerating electrode and an electron trap which produce a beam of electrons extending transversely across the chamber 1, in a well known manner. Two magnets 12 are mounted diametrically opposite each other on the inner surface of the body. portion 2 and produce a magnetic field which collimates the electron beam.

The ionisation chamber 11 is mounted on a metal plate 13 which is supported on rods 14. Each rod 14 is formed [from a conductor 15 surrounded with suitable insulation. The rods 14 also support two plates 16, 17 spaced apart to define a slit 18. The conductors .15 are connected respectively to high tension conductors 6 and the plates 16, 17 may thereby be maintained at different potentials.

The other electrodes of the ionisation chamber assembly are also supplied from the high tension. conductors 6 through conductors 19.

An aperture 21 is tor-med in the wall of the body portion 2 and a sample may be inserted into the ion source through this aperture. In operation of the ion source, the sample to be analysed is inserted through the aperture 21 into the'region of the ionisation chamber 11 and molecules of the sample material are ionised by bombardment by the electron beam. By means of a'plate suitably located in the ionisation chamber assembly, these ions are accelerated through the slit .18 towards the end wall 3 of the chamber 1.

The end wall 3 is formed with a channel 22 leading into a larger channel 23 which connects with the analyzer section of the mass spectrometer. The channel 23 is adapted to be closed by a ball valve 24 which is controlled by a plunger 25 extending at an angle to the axis of the channel 23 and co-operating with a seating in the channel.

The end of the channel 22 connecting with the chamber 1 is covered with a variable slit device 26. This device 26 is controlled by a plunger 27 by means of a micrometer 28 located outside the chamber 1. The micrometer 28 includes a longitudinally moving shaft 29 which is position.

The device 26 is illustrated on a larger scale in FIGS. 2 and 3, and comprises two substantially circular flat plates 31, 32 spaced apart by three flat segmental plates 33, 34 and 35, and two sliding plates 36, 37 whichare arranged to slide in the channel formed between the flat plates. The adjacent edges of the two sliding plates 36,37 are formed with chamfered projections so as to form two knife edges which extend across two apertures 38, 39

FIG. 5 is a side view sectioned on the plane VV of 7 FIG. 4;

FIG. 6 is a side view sectioned on the plane VIVI of FIG. 4; and

FIG. 7 is a plan view of a further variable slit device.

With reference to FIG. 1 the mass spectrometer ion source comprises a chamber 1 having a body portion 2, an end wall 3 and a cover plate 4. The end wall and the cover plate are vacuum tight sealed to the body portion in any well known manner. A duct 5 is provided in the body portion for evacuating the chamber and a plurality of high tension conductors 6 project through insulating bushings 7 in the cover plate 4.

The ionisation chamber assembly is supported from the cover plate 4 on two insulating supports 8, 9. The assembly comprises a box 11 in which is mounted a filaformed in the centers of plates 31, 32 respectively.

Outwardmovement of the two sliding plates 36, 37 in the channel is resisted by two leaf springs 41, 42 which extend across the channel. The plunger 27 extends through the space between the two smaller segmental plates 34, 35. The inner end of the plunger 27 is tapered and shaped so as to engage closely with the chamfered surfaces at the adjacent edges of the sliding plates, so that as the plunger 21 is forced inwardly the two sliding plates 36, 37 are forced apart. A spring 43 resists inward movement of the plunger 27.

The adjacent edges of the sliding plates 36, 37 control the width of the slit provided by the device 26 and the width of this slit controls the beam of ions emitted from the ionisation chamber described above. The slit defined by the device 26 is preferably aligned with the slit 18 between the fixed plates 16, 17.

By suitably calibrating the scale of the micrometer 28,

. apart.

the plates also move to the left 3 the width of the slit provided by the device 26 can be directly noted and varied as required.

The above described variable slit device 26 provides a slit the width of which can be varied from zero to about 0.03 inch in order to satisfy the requirements of the mass spectrometer.

The plates 36, 37 move simultaneously by equal amounts so that the center line of the slit remains stationary and it is not necessary to alter the deflection and focusing of the ion beam for each vibration in the slit width.

For satisfactory operationof the device it is essential that the plates should move so that the edges of the slit remain parallel, the slit width should be adjustable accurately and easily to fine limits, for example, within 0.0001 inch, and the whole device should be capable of with standing baking to a high temperature, for example, 300 C.

It is found diflicult in practice to make the parts so accurately that when the device is assembled the plates move freely and the sides of the slit'are parallel. Also when an apparently satisfactory slit device is placed in the ultra clean high vacuum conditions of an ion source a form of cold welding occurs and this causes the plates to stick after relatively few operations.

This latter problem is partially overcome by making the fixed and moving parts of the device of dissimilar metals, for example, stainless steel and hard chrome and using "molybdenum disulphide asa lubricant. Such a slit-device is satisfactory at room temperatures, but the moving parts still tend to stick after baking.

Two alternative forms of the variable slit device can be used to overcome the above difficulties and these are illustrated in FIGS. 4, 5 and 6 and FIG. 7 respectively.

With reference to FIGS. '4, Sand 6 the variable slit device comprises a fixed plate 51 with a central aperture 52, two movable plates 53,. 54, two tapered guide plates 55,56, two resilient retaining plates 57, 58, and a plunger 59 adapted to be moved by a mechanism located outside the ion chamber as described above.

The plate 51 extends over the wall of the ion chamber so that the aperture 52 is aligned with the channel 22.

The guide plates 55, 56 and the resilient plates 57, 58 are secured to the plate 51 by means of screws 61. The

movable plates 53, 54 are free to move over. the surface of plate 51 but their movement is constrained by theguide members 55, 56 and the plates are retained in position against plate 51 by the resilient plates 57, 58. The adjacent edges of the plates 53, 54 are partially chamfered at 62, 63 so as to form two knife edges bounded by supporting shoulders. V

Two springs '64, 65 anchored respectively by studs 66, 67 bias the movable plates 63, 64 resiliently. These springs force the plates towards the left in FIG. 4 and apart so that the plates remain in contact with the guide] members 55, 56. Therefore if the plates are moved to the In order to reduce the frictional losses between relatively moving components and the tendency to weld, the movable plates 53, 54 are made from stainless steel or a copper aluminum alloy while the fixed components are made of a suitable nickel-chromium alloy 'and the resilient plates 57, 58 and the springs 64, 65 are made of molybdenum.

In an alternative arrangement the hairpin springs 64, 65 are replaced by helical springs and the mechanical conneotion between the plunger 59 and the movable plates 53, 54 is modified by providing a peg on each movable plate cooperating with a 90 wedge bn' the plunger.

With the above design the problem of matching close limits on the movable plates 53, 54 and the guide members 55, 56 has been removed. The components need not be lubricated. 7

FIG. 7 illustrates a further alternative design of variable slit device for use in the ion source. This design is similar to that described with reference to FIGS. 4, 5

and .6 with the modification that all relatively moving parts are spaced apart by sapphire balls or rollers so as ,to prevent welding and provide easier relative movement.

77, 78 and two resilient retaining plates 79, 81 (81 shown partially cut away) secured to plate 75 by screws 82, and two movable plates 83, 84. The adjacent edges of the movable plates 83, 84 are partially chamfered at 85, 36'

so as to provide knife edges which define the size of the slit and are bounded by supporting shoulders. 7

Two springs 87, 88 bias the movable plartes 83, 84 againstthe guide members so that as the plates move to the right in FIG. 7 they tend to move apart. I v

A plunger 92 slides between two fixed guide members 93, 94 secured to plate 75 and is located by a pin 95 extending through .a slot 96 in the plunger. The end of the plunger is V-shaped and abuts the ends of the movable plates 83, 84.

The moving members 83, 84, and 92 are spaced apart from the guide members 77, 78 land 93, 94 respectively by sapphire balls or rollers 97. These balls or rollers are located in suitable grooves or slits. Conveniently V-shaped or circular recesses 98 are formed in guide plates 77, 78 while V-shaped grooves 99 are formed in the plunger 92, and conical or spherical recesses in guide plates 93, 94. These balls or rollers prevent direct contact between the relatively moving surfaces and hence reduce the frictional losses and prevent the relatively moving members from welding together. The balls or rollers do not necessarily roll relative to both relatively moving parts but need only provide sliding contact.

Blocks or rollers of sapphire or similar material may' alsobe used to separate the relatively moving surface of right in FIG. 4 they move-apart and if the plates are not acted upon byoutside forces they tend to move to the left and be forced together by the tapered guiding surfaces,

The plunger 59, shown partly cutaway in FIG.. 4, is

7 connected by two pegs 68, 69 to a rod 71, which slides'in recess 72 in the plate 51. Peg '68 extends through a hole formed by two triangularrecesses 73, 74 cut respectively in the adjacent edges of the plates 53, 54. Longitudinal movement of the plunger 59 therefore causes plates 53, 54 .to move relative to the guide members 55, 56. As the plates move to the right in FIG. 4 the springs 64, and the taper of the guide surfaces causes the plates to move The chamfered portion 62, 63--move apart and the slitwhich is aligned with the then moved to the'left of and closet-together so as Therefore the width of define the edges of aperture 52 If the plunger is to reduce the width of the slit.

the slit depend-s directly on the position of the plunger 59 J and the position of this plunger is controlled from outside the ion chamber.

moving members 83, 84 and 92-and fixed members 75,

' scribed above can be-used in combination with any other method of ion generation, for example the sparkmethod, and are not limited to use with the electron bombardment method described.

It will also be appreciated that the small slit in the variable slit device provides the only communication between the chamber of the ion source and the channel'22 leading to the analyzer section of the mass spectrometer and this arrangement enables differential pumping between the ion source. and the channel 22 to be practised, resulting in different pressure existing in these two regions.

What we claim is:

- 1. A mass spectrometer ion source comprising a chamber adapted to be evacuated, means for producing a beam of ions from a sample located within said chamber, an end wall to said chamber, surfaces defining a unique channel leading through said end wall to the analyzer slit extending across said channel so as to define a unique passage into said channel, means for causing said beam of ions to pass through said slit and along said channel, guide means including two relatively inclined guiding surfaces against which said two moveable plates abut respectively, means for resiliently biasing said plates against their respective guiding surfaces, and means for moving said plates along their guide means so that they move relatively and vary the width of said slit, said moving means being located partially outside said chamber and extending through the wall of said chamber in a vacuum tight manner.

2. A mass spectrometer ion source comprising a chamber adapted to be evacuated, means for producing a beam of ions from a sample located within said chamber, an end wall to said chamber, surfaces defining a unique channel leading through said end Wall to the analyzer section of said spectrometer, two relatively moveable plates located so that their adjacent edges define a slit extending across said channel so as to define a unique passage into said channel, means for causing said beam of ions to pass through said slit and along said channel, guide means formed from a suitable nickel chromium alloy and including two relatively inclined guiding surfaces against which said two moveable plates abut respectively, said plates being made of a material having a low frictional contact with the metal of the guide means, means for resiliently biasing said plates against their respective guiding surfaces, and means for moving said plates along their guide means so that they move relatively and vary the width of said slit, said moving means being located partially outside said chamber and extending through the wall of said chamber in a vacuum tight manner.

3. The structure of claim 2 wherein the plates are of stainless steel.

4. The structure of claim 2 wherein the plates are of a copper aluminium alloy.

5. A mass spectrometer ion source comprising a chamber adapted to be evacuated, means for producing a beam of ions from a sample located within said chamher, an end wall to'said chamber, surfaces defining a' unique channel leading through said end wall to the analyzer section of said mass spectrometer, two relatively moveable plates located so that their adjacent edges define a slit extending across said channel so as to define a unique passage into said channel, means for causing said beam of ions to pass through said slit and along 7 References Cited by the Examiner UNITED STATES PATENTS 2,674,698 4/54 Danforth et al. 250-413 2,852,684 9/58 Payne 25041.9 2,878,387 3/59 Chesterman 25041.9 2,961,538 11/60 Bishop 250-419 RALPH o. NILSON, Primary Examiner. 

1. A MASS SPECTROMETER ION SOURCE COMPRISING A CHAMBER ADAPTED TO BE EVACUATED, MEANS FOR PRODUCING A BEAM OF IONS FROM A SAMPLE LOCATED WITHIN SAID CHAMBER, AN END WALL TO SAID CHAMBER, SURFACES DEFINING A UNIQUE CHANNEL LEADING THROUGH SAID END WALL TO THE ANALYZER SECTION OF SAID MASS SPECTROMETER, TWO RELATIVELY MOVEABLE PLATES LOCATED SO THAT THEIR ADJACENT EDGES DEFINE A SLIT EXTENDING ACROSS SAID CHANNEL SO AS TO DEFINE A UNIQUE PASSAGE INTO SAID CHANNEL, MEANS FOR CAUSING SAID BEAM OF IONS TO PASS THROUGH SAID SLIT AND ALONG SAID CHAN- 